One of many extensively used non-volatile magnetic storage devices is a magnetic disk drive that includes a rotatable magnetic disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. The suspension arm is supported by an actuator that can swing the suspension arm to place the slider with its air bearing surface (ABS) over the surface of the magnetic disk.
When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes the slider to fly on a cushion of air at a very low elevation (fly height) over the magnetic disk. When the slider rides on the air, the actuator moves the suspension arm to position the assembly of write and read heads over selected data tracks on the magnetic disk. The write and read heads write and read data in the selected data tracks, respectively. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions, respectively.
The write head includes a magnetic write pole and a magnetic return pole that are magnetically connected with each other at a region away from the ABS, and an electrically conductive write coil surrounding the magnetic write pole. In a writing process, the electrically conductive write coil induces magnetic fluxes in the magnetic write pole. The magnetic fluxes form a magnetic write field emitting from the magnetic write pole to the magnetic disk in a direction perpendicular to the surface of the magnetic disk. The magnetic write field writes data in the selected data tracks, and then returns to the magnetic return pole so that it will not erase previously written data in adjacent data tracks.
The read head includes a read sensor that is electrically connected with lower and upper ferromagnetic shields, but is electrically separated by insulation layers from longitudinal bias layers in two side regions. In a reading process, the read head passes over data in a selected data track, and magnetic fields emitting from the data modulate the resistance of the read sensor. A change in the resistance of the read sensor is detected by a sense current passing through the read sensor, and is then converted into a voltage change that generates a read signal. The resulting read signal is used to decode data in the selected data track.
A tunneling magnetoresistance (TMR) read sensor is typically used in the read head. The TMR read sensor includes a nonmagnetic insulating barrier layer sandwiched between a ferromagnetic reference layer and a ferromagnetic sense layer. The thickness of the barrier layer is chosen to be less than the mean free path of conduction electrons passing through the TMR read sensor. The magnetization of the reference layer is pinned in a direction perpendicular to the ABS, while the magnetization of the sense layer is oriented in a direction parallel to the ABS. When passing the sense current through the TMR read sensor, the conduction electrons are scattered at lower and upper interfaces of the barrier layer. When receiving a magnetic field emitting from data in the selected data track, the magnetization of the reference layer remains pinned while that of the sense layer rotates. Scattering decreases as the magnetization of the sense layer rotates towards that of the reference layer, or increases as the magnetization of the sense layer rotates away from that of the reference layer. This scattering variation induces a TMR effect characterized by a change in the resistance of the TMR read sensor in proportion to the magnitude of the magnetic field and cos θ, where θ is an angle between the magnetizations of the reference and sense layers. The change in the resistance of the TMR read sensor is then detected by the sense current and converted into a voltage change that is processed as a read signal.
The TMR read sensor has been progressively miniaturized for magnetic recording at higher linear and track densities. To increase linear densities, its thickness, which defines a read gap, is reduced by utilizing thinner reference, barrier, sense or other layers. To increase track densities, its width, which defines a track width, is reduced by patterning with an advanced photolithographic tool. In addition, to increase signal sensitivity for compensating signal losses caused by the reductions of its thickness and width, its length, which defines a stripe height, is also reduced by deeper chemical mechanical polishing.
In this miniaturized TMR read sensor, a junction area (RJ) defined by its width and length becomes so small that its junction resistance (AJ) will be too high to maintain low electronic noises and prevent electrostatic discharges. In addition, a junction volume defined by its width, length and thickness becomes so small that its magnetic excitation will be too high to maintain low magnetic noises. In this sensor miniaturization progress, it is challenging to maintain the electrical resistance of less than 400Ω for minimizing electronic noises and preventing electrostatic discharges, and to ensure a pinning field of more than 1,200 Oe for minimizing magnetic noises though rigidly pinning the magnetization of the reference layer. In any approaches to minimizing the electrical and magnetic noises, it is also challenging for the TMR read sensor to maintain a high TMR effect and thus to perform the magnetic recording with a high signal-to-noise ratio.